Dr. Duncan "Om" MacDougall (c. 1866 – October 15, 1920) was an early 20th-century physician in Haverhill, Massachusetts who sought to measure the mass lost by a human when the soul departed the body at death. MacDougall attempted to measure the mass change of six patients at the moment of death. His first subject, the results from which MacDougall felt were most accurate, lost "three-fourths of an ounce", which has since been popularized as "21 grams".[1]

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In 1901, MacDougall weighed six patients while they were in the process of dying from tuberculosis in an old age home. It was relatively easy to determine when death was only a few hours away, and at this point the entire bed was placed on an industrial sized scale which was reported to be sensitive to "two-tenths of an ounce". He took his results (a varying amount of unaccounted for mass loss in four of the six cases) to support his hypothesis that the 'soul' had mass, and when the 'soul' departed the body, so did this mass. The determination of the 'soul' weighing 21 grams was based on the loss of mass in the first subject at the moment of death.

MacDougall later measured fifteen dogs in similar circumstances and reported the results as "uniformly negative," with no perceived change in mass. He took these results as confirmation that the 'soul' had weight, and that dogs did not have 'souls'. MacDougall's complaints about not being able to find dogs dying of the natural causes that would have been ideal led one author to conjecture that he was in fact sacrificing the experimental animals, as is standard practice in scientific experiments.[2] On March 10, 1907, before MacDougall was able to publish the results of his experiments, New York Times broke the story in an article titled "Soul has Weight, Physician Thinks". MacDougall's results were published in April of the same year in the Journal of the American Society for Psychical Research and the medical journal American Medicine.

Researchers have revealed that MacDougall's experimental results were flawed, due to the limitations of the available equipment at the time, a lack of sufficient control over the experimental conditions, and the small sample size.

When MacDougall’s findings were published in the New York Times in 1907 fellow physician Augustus P. Clarke had a field day. Clarke noted that at the time of death there is a sudden rise in body temperature due to the lungs no longer cooling the blood, and the subsequent rise in sweating could easily account for MacDougall’s missing 21 grams. Clarke also pointed out that dogs do not have sweat glands (thus the endless panting) and so it is not surprising that their weight did not undergo a rapid change when they died.[3]

Science writer Karl Kruszelnicki has noted that out of MacDougall's six patients only one had lost weight at the moment of death. Two of the patients were excluded from the results due to "technical difficulties", a patient lost weight but then put the weight back on and two of the other patients registered a loss of weight at death but a few minutes later lost even more weight. MacDougall did not use the six results just the one that supported his hypothesis. According to Kruszelnicki this was a case of selective reporting as MacDougall had ignored five of the results.[1]

The physicist Robert L. Park has written MacDougall's experiments "are not regarded today as having any scientific merit" and the psychologist Bruce Hood wrote that "because the weight loss was not reliable or replicable, his findings were unscientific."[4][5]